Jed Shannon, a young American movie star on location in England, has received a threat against his life. When the Hardys set out to investigate, they are drawn into a case worthy of Sherlock Holmes -- and a conspiracy as thick as the London Fog.

Jillian Seabright, a beautiful British actress befriended by Jed, has vanished. The key to the mystery lies in her resemblance to missing emerald heiress Emily Cornwall. Frank and Joe trace the damsels in distress to a medieval mansion on the moors -- Castle Fear. Dodging bullets and battle-axes, the boys are out to prove that chivalry is not dead -- but one wrong move and they will be!